Ask Dr Bob: What is the control objective?
Video Transcript
Hello, I’m Bob Rice, and I’m the Vice President of Engineering for Control Station. Today, we’re going to be covering the control objective, which is the purpose behind the PID controller, what is its goal? What is its objective while trying to maintain control? What is good control? Well, good control is simple. S I M P L E. No, this is not a spelling class, this is an acronym for what good control should accomplish.
Number one, it should accomplish good safety. Okay, what is good control, good control is a safe process, it’s making sure the level stays where it needs to the temperatures stay where they need to, to maintain safe efficient control, you don’t spill material onto the ground, you don’t exceed tolerances pressure, tolerances, temperature tolerances of your vessels, you want to make sure it’s safe.
The next is impact. Impact of this loop on the surrounding systems, a lot of loops are interacting with each other. They don’t live as individual islands of automation that don’t touch other parts of your factory. If you had an oven or a furnace that had multiple heating zones within it, each of those may have a temperature controller associated with it. If one temperature controller starts to swing, they all start to swing. Okay, so you have to understand how each of the inter-individual controllers impact each other.
The next is management. And I’m going to be honest, I needed something for the letter M. And everybody likes to incorporate management into their decisions, whether they like to or not, right management is what is the goal of that loop? There are usually constraints and tolerances that are designed by management or the process engineers associated with that piece of equipment, certain tolerances, around setpoint, certain tolerances around flow or pressures, and so forth. That’s got to be part of the control objective. are you operating 100 degrees or 1000 degrees, the process is going to behave differently.
The next and the most important is profit. Okay? Profit, ours where your money is made for that loop? Okay, if you’re using an expensive material, you want to make sure you use the most of it. Is that particular loop a bottleneck in your process? Does it need to respond as quickly as possible? Because if this unit is down longer than anything else, that’s going to cause you to lose money? You got to make sure you incorporate profit? And what is the most important aspect of the controller? And the objective is how much money it’s going to make you.
Okay, the next is longevity. How often do you actually have to go back to this loop and actually, retune it? Is it something that’s constantly changing dynamics, and you’re constantly having to go back to this loop and make adjustments and make changes, longevity is associated with the non-linearity or the stability of the process? Does it constantly change, and every six weeks you have to go back to it and make adjustments?
The last example here is equipment. Okay? The equipment would be things like your valves, your VFDs, and so forth, that are actually driving the process. These things are expensive. If your sensors are expensive, and you have membranes on them that cannot handle changes in flow and changes in pressure that are extreme, you got to make sure you take that into account with your control objective, because this is going to allow you to decide whether or not you want to tune the controller to be fast or slow. Or somewhere in between, right? Because good control depends on the objective that you’ve defined.
If you do not have an objective, what are you tuning towards? What is your goal? Okay, so let’s talk about one particular example. And this particular example is a level controller on a reflux drum for the top of a column. So let’s talk a little bit about what the control objective is, and the purpose behind the system itself. So we’re going to start with a distillation column over here on the left, and we’re going to be taking the vapor out of the top, running it through a condenser, and we’re going to drop that liquid that gets condensed into a reflux drop. The reflux drum then has a split output, some of it goes back to the column and some of it goes further downstream to another unit. Well, we have to maintain liquid level inside of this. So we apply a level control We’re measuring the liquid level. And to control it, we’re actually adjusting the reflux, or the flow back into the column. If the level liquid level gets too high, we open the valve gets too low, we close the valve, very simple.
But what’s the objective? What’s the point behind this level controller, the point is, we want to make sure the liquid level doesn’t get too high, that we start to get into a danger zone where we may have to flare or something like that, that the vapor which is wasting product as well as environmental emissions, the others, we want to make sure it doesn’t get too low, that we don’t have enough product go to the next downstream unit, because that may cause downstream problems, right, you may end up tripping a unit if it doesn’t have enough feed or something like that, you want to make sure the liquid level is between these tolerances. But the objective isn’t to maintain a perfectly flat, stable, perfect setpoint process variable to the setpoint. The goal is actually just as long as you’re between these two constraints, everybody’s happy. But we also want to make sure this control valve here doesn’t move around too much. Because if the flow going into this column, this reflux going back into this column starts to move around and oscillate, that’s going to disrupt the stability of this column, which actually disrupts the vapor equilibrium at the top of the column, which causes the vapor flow out of the top to oscillate, which means that any movement in this control valve could actually propagate and recycle back to the drum itself. This means that now any excess movement of control valves is going to cause more movement in my process variable, which is going to cause more movement on my control valve, and you end up with a very kind of unstable system. So the purpose of this controller is to maintain a liquid level between these with as minimal controller output movement as necessary. So in this case, you actually want to tune this controller to be relatively slow, just to be able to maintain the liquid level between those constraints, and nothing more. It doesn’t have to be ultra-fast.
We know that because we define the objective, we defined what was safe, right? We define the impact of the system, we define the whims of the management associated with it, right? We understand the economic and the profit associated with this system. We understand the longevity, we understand the equipment associated with it. We’ve defined those so now we can tune the controller to be effective.
If you have a particular topic or an idea that you would like us to cover, please email us at askus@controlstation.com. Thank you, and I hope you enjoy this video series.